A condenser microphone comprises a microphone unit having a diaphragm and a fixed pole arranged opposite each other. Since the microphone unit has a very high impedance, the condenser microphone also contains an impedance converter. The impedance converter is normally composed of an FET (Field Effect Transistor) but in rare cases, of a vacuum tube.
An electronic circuit for audio output from the condenser microphone is mounted on a substrate, which is then housed in a microphone case consisting of a metal cylinder. Normally, a connector housing section is provided at one end of the microphone case. A 3-pin type output connector (3-pin output connector) defined in EIAJ RC5236 (latch lock type round connector for audio equipment) is installed in the connector housing section.
In operation, the output connector is connected to a phantom power source via an output cable (balanced shield cable) having a connection plug to which the output connector is connected. The connection plug comprises three female contacts corresponding to the three pins (ground, a hot side of a signal, and a cold side of a signal) of the output connector, and a metal outer ring connected to a shield coated wire. When inserted into the connector housing, the outer ring contacts an inner surface of the connector housing to electrically connect to the microphone case.
When an intense electromagnetic wave emitted by, for example, a cellular phone is applied to the microphone or output cable, it may be transmitted through the output cable to enter the microphone via the output connector. Then, the electromagnetic wave is demodulated by the impedance converter and output by the microphone as noise of an audible frequency.
To prevent this, a grounding first pin, one of the three pins of the output connector, is connected to the microphone case. Further, the outer ring of the connection plug connected to the shield coated wire of the outer cable is contacted with and electrically connected to the inner surface of the microphone case (the inner surface of the connector housing section). The microphone case is thus provided with a shield function.
However, since the connector housing section and the outer ring of the connection plug are subjected to cylindrical female-male fitting, they are contacted with each other at a point. The point contact portion has an impedance for a high frequency to make the shielding incomplete. Further, if the connector housing section and the outer ring of the connection plug are loose owing to a dimensional error or the like, the contact point cannot be established, thus making the shielding unstable. Such incomplete and unstable shielding results in noise attributed to the electromagnetic wave. Further, the looseness of the fitted portion may result in vibration noise.